The present invention relates to a head positioning device for use in a magnetic disk equipment, and more particularly to a head positioning device which includes an inexpensive circuit for compensating for an effect of an external force on the positioning of a magnetic head, to improve the positioning accuracy of the magnetic head and to shorten an access time to a desired track.
In a general magnetic disk equipment, a magnetic head is lifted with a increase in the rotational speed of a magnetic disk. Similarly, the head is lowered with a decrease in rotational speed. In this case, in order to protect the data recording area of the magnetic disk, it is necessary to place the magnetic head outside of the data recording area when the rotation of the magnetic disk is started or stopped. Hence, an actuator mounted with the magnetic head is usually pulled in a predetermined direction by a retracting spring so that the magnetic head or heads of the device are located at the innermost of the magnetic disks or disks when the actuator is not operated. Thus, an external force is applied to the actuator by the retracting spring. Furthermore, the mechanical resistance of a flexible printed circuit (FPC) for exchanging signals with the moving part of the actuator and the wind pressure due to the rotation of the magnetic disk act as additional external forces applied to the actuator. These external forces will reduce the positioning accuracy of the magnetic heads.
In recent years, in order to increase the capacity and operating speed of a magnetic disk equipment, the track width of a magnetic disk and the weight of an actuator have been greatly reduced. As a result, the effect of the external forces applied to the actuator has became too large to be neglected. Accordingly, it is usual for a magnetic disk equipment to provide compensation for the effect on the positioning of the magnetic heads by external forces applied to the actuator.
A method of compensating for the effects of the external forces has hitherto been known, in which the external forces applied to an actuator are compensated for by using a constant correction value, independently of which of tracks on a magnetic disk confronts a magnetic head. In a case where the actuator is pulled by a retracting spring in a direction from the center of the magnetic disk toward an outer track, however, an external force which is applied to the actuator at a time the magnetic head is placed on an inner track, is stronger than an external force applied at a time the magnetic head is placed on an outer track, and hence the above constant correction value method cannot make full compensation for the external forces across the entire surface of the disk.
Another method has been proposed, in which an appropriate correction value for each of tracks between an innermost track and an outermost track is selected from a predetermined correction table. In this method using the correction table, however, it is impossible to compensate for a change in gravity applied to an actuator due to variations in the structure of a magnetic disk equipment, or caused by whether the magnetic heads are arranged in a horizontal direction or in a vertical direction.
A further method of solving the above problem has been proposed, in which the measured value of an external force is obtained with the aid of an A-D converter, and the correction value is updated on the basis of the measured value, as described in, for example, a Japanese patent application JP-A-58-29180. In more detail, the measured value of an external force is obtained in such a manner that a current for driving an actuator is detected, and the detected current value is converted by the A-D converter into a digital value. In this method, a relatively expensive A-D converter is required. In addition also the measured value is affected by an offset voltage generated in a phase compensation circuit or other circuits. Thus, it is not always possible to obtain an accurate correction value.
Still another method has been proposed, in which an analog feedback loop is formed so that an external-disturbance cancelling signal is applied to a power amplifier, as described in Japanese patent applications JP-A-59-146486 and JP-A-59-146485. In the above method of compensating for external forces by the real time processing using an analog circuit the external forces are composed of low-frequency components approximate to a D.C. component. As a result it takes a lot of time to feed a change in external force back to the power amplifier after the change has been detected. That is, the above method is disadvantageous in its response characteristics.
In a system for controlling the positioning of a magnetic head, a feedback loop is usually formed on the basis of a position signal, and moreover phase compensation is carried out to improve the stability of the feedback loop and to reduce a steady-state deviation. However, when a low-frequency gain is made too large, follow-up characteristics are improved but it takes a lot of time for the position signal to reach a predetermined level after the magnetic head has been moved. Hence, it is undesirable to make the low-frequency gain greater than a predetermined value. Accordingly, when an external force is applied to an actuator, the level of a head-position signal will be changed by an amount corresponding to the steady-state deviation.